Gold electroplating electrolytes



Dec. 29, 1970 DALTON ETAL 3,551,305

GOLD ELECTROPLATING ELECTROLYTES Filed June 23, 1967 m Quoo m9xoma z 22mm??? to mhm jmm L 7. 3074 8529630 332?. Ill

THROWING POWER.

EFFICIENCY Ill M31233. uozorfijow ZQELIOm m zofijom zomE mzoo mzoEbJow zomEgzou United States Patent US. Cl. 204-46 6 Claims ABSTRACT OF THE DISCLOSURE An electrolytic gold plating solution comprising .5 to 25 grams per litre of gold in the form of a soluble gold salt, 20 to 450 grams per litre of the sodium salt of N,N-di-2-hydroxyethylglycine or an amino acid, 30 to 1600 grams per litre of an aliphatic dicarboxylic acid and 15 to 800 grams per litre of an alkali metal hydroxide.

This invention relates to an electrolytic gold plating solution and to a plating process utilizing the solution.

The objectives, features, and advantages of the invention will be apparent from the following detailed description taken in conjunction with appended claims and attached drawings in which:

FIG. 1 is a graph indicating the pH stability of the solution.

FIG. 2 is a graph showing a comparison of the cathode efiiciency and throwing power of the solution with other plating solutions.

The invention provides a gold plating solution containing:

(a) 0.5 to 25, preferably 5 to 7.5 grams per litre of gold, in the form a soluble gold salt, preferably potassium aurocyanide (K Au[CN] (b) 20 to 450, advantageously 25 to 100, preferably about 45 grams per litre of a brightener that will complex iron, cobalt and nickel, advantageously a hydroxylated amino acid, especially an amino acid that contains only H, C, O and N atoms in the molecule, for example, glycine, alanine, valine, leucine, serine, lysine, or a salt, preferably an alkali metal or alkaline earth metal salt, of such an acid, the brightener being, for example sodium salt of N,N-di-2-hydroxyethyl-glycine;

(c) 30 to 1600, for example, 30 to 600, preferably about 50, grams per litre of an aliphatic dicarboxylic acid v that will form a water-soluble complex with gold, preferably succinic acid; and

"(C1) to 800, preferably about 50, grams per litre of an alkali metal hydroxide, preferably potassium or sodium hydroxide, the solution having a pH within the range of 3.5 to 12.0 preferably 6 to 6.2, and a density within the range of 1 to 33 B=., preferably 6 to 12 B6.

The plating process using the solution of the invention may be operated at temperatures within the range of room temperature (25 C.) to 75 C., preferably 65 C. to 70 C. at a current density ranging from below 1 to 40 amperes per square foot and gives excellent results when the proportions of the constituents of the solution and the conditions are varied within the defined ranges. The deposit obtained is smooth, ductile, bright to semi-matt, depending on the pH of the solution, 24 carat gold having a hardness of about 65 DPH, temperature resistant properties that are about twice as good as those of cyanide Patented Dec. 29, 1970 ice gold and high resistance to chemical attack, especially acid, for example, nitric acid, attack.

The stability of the solution is very high and there is no breakdown noticeable even after working periods of up to two years, addition of material to the solution only being necessary to make gOOd drag out.

The solution exhibits very good throwing power and the cathode efficiency of the bath is 85 to 90%. A further advantage of the solution is that it will not co-deposit iron, cobalt and nickel, thus producing a very pure deposit.

The following example illustrates the invention:

A plating solution was made up by dissolving 6 grams per litre of potassium aurocyanide, 50 grams per litre of potassium hydroxide, 54 grams per litre of succinic acid and 90 millilitres per litre of a 47% by weight aqueous solution of the sodium salt of N,N-di-2-hydroxyethylglycine in water. The solution had a density of 8 B. and a pH of 6.0.

The solution was used as the electrolyte in a glass plating tank using platinized titanium anodes and an operating temperature of C. to C. A KOVAR header cathode can be used. The throwing power of the solution was and the efficiency was up to The properties of the solution are shown in the accompanying drawings, FIG. 1 of which is a graph indicating the pH stability of the solution and FIG. 2 of which is a graph showing a comparison of the cathode efiiciency and throwing power of the solution with other plating solutions.

Referring to FIG. 1, which is a plot of pH of the solution against the number of pellets of potassium hydroxide added to the solution, it can be seen that after an initial sharp rise in the pH, the slope of the curve increases greatly after a pH of about 5.2 is reached and that for the addition of a large amount of alkali only a very small charge in the pH of the solution occurs, thus obviating the need for frequent pH adjustment of the solution during operation.

FIG. 2 shows a comparison between the throwing power and efiiciency of six proprietary plating solutions (A to F), a cyanide solution and the solution of the example. It will be seen that none of the comparison solutions has a throwing power as high as the solution of the invention coupled with the high efiiciency of the solution of the invention.

What is claimed is:

1. An aqueous electrolytic gold plating solution comprising .5 to 25 grams per litre of gold in the form of a soluble gold salt, 20 to 450 grams per litre of the sodium salt of N,N-di-2-hydroxyethylglycine, 30 to 1600 grams per litre of a aliphatic dicarboxylic acid and 15 to 800 grams per litre of an alkali metal hydroxide, said solution having a pH of 3.5 to 12.0.

2. The electrolytic gold plating solution according to claim 1 wherein said soluble gold salt is potassium aurocyanide.

3. The electrolytic gold plating solution according to claim 2, wherein said aliphatic dicarboxylic acid is succinic aid.

4. The electrolytic gold plating solution according to claim 3, wherein said alkali metal hydroxide is potassium hydroxide.

5. An aqueous electrolytic gold-plating solution comprising 5 to 7.5 grams per liter of gold in the form of a soluble gold salt, 25 to 100 grams per liter of a brightener selected from the group consisting of a hydroxylated amino acid and a hydroxyalkylated amino acid, 30 to 600 grams per liter of aliphatic dicar-boxylic acid, and 15 to 800 grams per liter of an alkali metal hydroxide, and having a pH of 3.5 to 12.0.

6. A solution as defined by claim 5, wherein said souble gold salt is an alkali metal aurocyanide.

References Cited UNITED STATES PATENTS 2,765,269 10/ 1956 Ostrow et a]. 204-46 3,123,484 3/1964 Pokras et al. 117130 X 3,380,814 4/1968 Cathrein et a1 204-56X FOREIGN PATENTS 939.007 10/ 1963 Great Britain 204-46 10 GERALD L. KAPLAN, Primary Examiner 

